Stairs, and how they work.
I’m going to begin by telling you some things even many of us professionals either overlook or do not understand.
Your weight is not supported by the crispness nor by the accuracy of the fancy cuts involved in making a stinger. The only part of a stringer that carries weight is the continuous part remaining BELOW the cuts. That slim, narrow strip of lumber you see running the length of the whole staircase is all that is keeping the full stairway from coming down.
This is one reason all the impatient carpenters who make their stringer cuts completely with a circular saw (not finishing off the cuts cleanly and squarely with a hand saw) are dead wrong in doing so. That extra couple of inches of saw kerf taken from the lower portion of the stringer weakens it.
In most areas of our country there are no existing building codes for this vitally important part of the stringer. The height of the individual steps (the ‘RISE’) is taken into consideration. The width of the TREAD upon which your foot rests (the ‘RUN’) is also dictated. But not the meat of the stringer that supports all the weight.
In general, it is an accepted practice to assure that at least 5-1/2” of material is left in a stringer after the cuts are made. I personally feel it should be closer to six inches, or more. Code only calls for 5".
Nor do many municipalities stipulate how many stringers to use in constructing an average staircase. Did you know that almost all normal stairways built in the older houses of the past have only two (2) stringers? This is an important fact to absorb. There was a reason for it.
Using more than two stringers sounds good… at first. But it often causes more problems than you might expect. You see, there is very little leeway for any error or discrepancies in the dimensions of each stringer used in a set of stairs.
A single stair tread will sit comfortably on just two stringers even if they are not cut ‘exactly’ identical. Add a center stringer and everything changes. If that center stringer is cut even a sixteenth of an inch higher or larger across the space where a given tread will sit, that tread will have a tendency to ‘rock’ on the center stringer. It will not rest equally on all three stringers unless they are all three perfectly identical in every measurement and angle. This seldom happens. Erroneous measurements, slightly different cuts, the natural ‘crowning’ of different pieces of lumber used… all contribute to the likelihood that those three stringers will not turn out to be identical to one another.
Even if you COULD guarantee that all the stringers were perfect matches, you have yet another couple of problems to deal with. The installation of the stringers where they rest on the floor and against the upper framing members has to also be perfectly aligned. Again, this doesn’t often happen in the real world. Secondly, if there is any bowing of the tread lumber, it cannot sit flatly on three or more stringers.
So, on a normal three or four foot wide stairwell, two stringers are better than three unless the center stringer has been cut a little smaller than the two side stringers, and adjusted with glued shims to tighten the fit to the treads. This is one more thing that usually is not done.
In most good stair work, the treads are made of sufficiently thick material to carry the weight of a person across the stairway span. This is usually all you need… It’s really tough to break a 2” thick board only three feet long.
Surprisingly, a part of the stair structure that helps to support your weight upon the tread is the riser. But only if properly installed. Improperly installed, a riser does little more than add extra weight to a stairway. Unfortunately, most carpenters are not cognizant of this. They consider the riser as nothing but a decorative addition to conform to the building codes.
The riser material is going to be from 6” to almost 8” high all the way across the length of the tread. That is a very strong piece of wood by itself… without the tread helping out. It will provide tremendous additional support to the edge of the tread above it right where your foot comes into contact with the tread. But, as I said, only if properly installed.
This effective installation involves little more than routing a dado (a groove) in the lower edge of the riser. This dado encases the full edge of the back of the lower tread in each set of two treads.
It is installed this way: (Follow along with the accompanying drawing)
Install the two stringers.
Place the first riser directly on the floor with the top edge even with the stringer tread cuts for the first step.
Cut the second riser 2-1/2” wider than the intended riser height, and cut a dado into it that will soon receive the back edge of tread #1. (This assumes 2" treads.... for simplicity)
Now install the riser for the second step. It nails right into the face of the stringers with the dado groove facing outward.
Put a bead of glue in the dado groove and on top of the stringer tread cuts
Slide the tread for the first step into the dado groove on riser #2.
Nail through riser #2 into the tread from the back (the inside of the stairwell)
Now nail the tread down to the stringer. (Yes, you could have nailed the tread down first if you chose to. This sequence really doesn’t matter much.)
The first step is now installed. You simply repeat this installation sequence all the way up.